1. Field of the Invention
The present invention relates to a deposited-film forming method of forming a thin film composed of an alloy on a substrate by use of an alloy target (a sputtering method) and, more particularly, to a method of forming a deposited film on a substrate by ionizing sputtering particles (an ionized sputtering method).
2. Related Background Art
In various types of semiconductor devices, alloy thin films are used as wiring layers and heater layers. As methods of forming such alloy thin films, there are available the resistive heating deposition method, the electron beam (EM) deposition method, the sputtering method, etc. Among others, the sputtering method that is favorable in terms of controllability and mass production is frequently used. Apparatuses for forming an alloy thin film using the sputtering method have problems such as the fact that the composition of an alloy thin film is apt to differ from the composition of an alloy target, and in order to solve such problems various measures have been required.
There is a multitarget simultaneous sputtering method as one of the conventional sputtering methods by which an alloy thin film is formed. In the multitarget simultaneous sputtering method, one target is provided for each metallic element of an alloy thin film, a sputtering power source is connected to each target, and a thin film is formed by simultaneously discharging electricity. Because in this multitarget simultaneous sputtering method input power can be adjusted for each target and besides the film forming rate is variable, it is possible to form an alloy thin film of an arbitrary composition and even during film forming the composition of an alloy thin film can be easily adjusted.
However, in carrying out this multitarget simultaneous sputtering method, it is necessary to use targets in the same number as the number of types of elements of the alloy thin film and accordingly it is necessary to increase the sizes of the target unit, sputtering power source and film forming chamber, posing the problem that the equipment cost increases substantially. Furthermore, in the multitarget simultaneous sputtering method, because of use of a plurality of targets, running costs also increase. Therefore, this method is not adopted as a mass production technique and is used mainly as an experimental technique.
There is an alloy sputtering method as another sputtering method of forming an alloy thin film. In the alloy sputtering method, one alloy target composed of the same elements as those of the alloy thin film is used. In this alloy sputtering method, it is possible to form an alloy thin film by use of one target and equipment cost and running costs are low. Therefore, this method is favorably used as a mass production technique.
However, in this alloy sputtering method, sputtering particles sputtered from the target collide against a sputtering discharge gas and sputtering discharge gas ions to scatter before becoming incident upon the substrate, whereby the composition of an alloy thin film deposited on the substrate differs from the composition of the target. More specifically, the scattering condition of sputtering particles differs in kinds of elements; among the elements constituting the target, the lighter the weight of an element, the greater the extent of scattering, and when an element is heavy, then the element will become less apt to scatter in proportion. Therefore, although the composition of sputtering particles has the same composition as that of the alloy target immediately when they sputter from the target, the composition of the alloy thin film finally deposited on the substrate is such that compared with the alloy target, light elements become small in its content and heavy elements become large in its content.
Furthermore, under the alloy sputtering method, also in a case where pressure is changed in order to change the film quality of an alloy film or in a case where the flow rate ratio of a sputtering discharge gas to a reactive gas is changed in reactive sputtering, the scattering condition of sputtering particle changes, thereby posing the problem that the composition of the alloy thin film changes. For this reason, in the alloy sputtering method, it is necessary to prepare beforehand a plurality of targets having different compositions and to create the conditions for obtaining an alloy thin film of the desired composition by forming an alloy thin film for each target. This creation of conditions required much time. Furthermore, in the alloy sputtering method, similarly much time was required also in changing film quality.
On the other hand, in Japanese Patent Publication No. 7-9061 discloses the plasma control magnetron sputtering method as a further sputtering method of forming an alloy thin film. In this plasma control magnetron sputtering method, first, a target of one element of the elements forming an alloy thin film is used as a base target and other element materials are arranged as small pieces in a plurality of regions on the base target. And a plasma is caused to move in the direction of radiation of the target by changing the value of a current flowing through a solenoid coil disposed in the peripheral portion of the target and changing the magnetic field distribution on the target surface, and an alloy thin film is deposited by causing sputtering while adjusting the composition.
However, the plasma control magnetron sputtering method has the following problems:                In order to form an alloy thin film of the desired composition, much time is required in creating conditions, such as the setting of the areas of the small-piece targets and the values of a current caused to flow through the solenoid coil.        In order to form an alloy thin film of the desired composition, it is necessary to continuously adjust the values of a current caused to flow through the solenoid coil until the target is completely consumed.        When for some reason the small-piece chips have moved, the composition of an alloy thin film changes.        
In view of the foregoing, it was difficult to adopt the plasma control magnetron sputtering method as a mass production technique.
As described above, the conventional methods of forming a deposited film had the following respective problems:
(1) In the multitarget simultaneous sputtering method, it is necessary to use targets in the same number as the number of types of elements of the alloy thin film. The sizes of a target unit, a sputtering power source and a film-forming chamber are increased. Therefore, the equipment cost is high.
(2) In the alloy sputtering method, the scattering condition of sputtering particles differs in the kinds of the elements. Among the elements constituting the target, the lighter the weight of an element, the greater the extent of scattering and if an element is heavy, then the element will become less apt to scatter in proportion. Therefore, although the composition of sputtering particles has the same composition as that of the alloy target immediately when the particles sputter from the target, the composition of the alloy thin film deposited on the substrate has a smaller number of light elements and a large number of heavy elements, compared with the alloy target, whereby the composition of the alloy thin film becomes different from the desired composition. Therefore, in order to form an alloy thin film of the desired composition, it becomes necessary to create conditions beforehand by use of a plurality of targets having different compositions and the creation of conditions required much time. Furthermore, similarly much time was required also in changing film quality.
(3) In the plasma control magnetron sputtering method, much time is required to create conditions necessary for forming an alloy thin film of the desired composition, such as the setting of the areas of the small-piece targets and the values of a current caused to flow through the solenoid coil, it is necessary to continuously adjust the values of a current caused to flow through the solenoid coil until the target is completely consumed in order to form an alloy thin film of the desired composition, and the composition of an alloy thin film changes when for some reason the small-piece chips have moved.